A conventional electrochemical cell 10 is shown in FIG. 1. Cell 10 may, for example, comprise a PEM (proton exchange membrane) fuel cell. Cell 10 has a manifold 12 into which is introduced a fuel, such as hydrogen gas. The fuel can pass through a porous current-carrying layer 13A into an anode catalyst layer 14A, where the fuel undergoes a chemical reaction to produce free electrons and positively charged ions (typically protons). The free electrons are collected by current-carrying layer 13A, and the ions pass through an electrically-insulating ion exchange membrane 15. Ion exchange membrane 15 lies between anode catalyst layer 14A and a cathode catalyst layer 14B. Cell 10 has a manifold 16 carrying an oxidant (e.g. air or oxygen). The oxidant can pass through a porous current-carrying layer 13B to access cathode catalyst layer 14B.
As shown in FIG. 1A, electrons travel from the sites of chemical reactions in anode catalyst layer 14A to current-carrying layer 13A. Protons (or other positively charged ions) travel into and through ion exchange membrane 15 in a direction opposite to the direction of electron flow. Electrons collected in current-carrying layer 13A travel through an external circuit 18 to the porous current-carrying layer 13B on the cathode side of cell 10. In such cells, electron flow and ion flow occur in generally opposite directions and are both substantially perpendicular to the plane of ion exchange membrane 15.
Catalyst layers 14A and 14B must be “dual species conductive” (i.e. they must provide conductive paths for the flow of both electrons and ions). Ion exchange membrane 15 must be single species conductive (i.e. it must permit ions to flow while providing electrical insulation to avoid internal short-circuiting of cell 10).
Many electrochemical devices include some form of porous conductive reactant diffusion media to carry current away from a catalyst layer. This compromises the ability to transport reactants to the catalyst sites, and introduces a difficult material challenge. Further, there are manufacturing and cost issues associated with the inclusion of reactant diffusion layers. A major problem in designing high performance electrochemical cells is to provide current-carrying layers which permit current to be passed into or withdrawn from the cell while permitting reactants to enter the cell and products of the reactions to be removed from the cell.
Despite the vast amount of fuel cell research and development that has been done over the past decades there remains a need for more efficient electrochemical cells that can be produced cost effectively and which provide improved access for reactants to the electrochemical reaction sites.